According to “Cellular Materials,” Encyclopedia of Polymer Science and Engineering, vol. 3, pages 1-59, (2d ed. John Wiley & Sons, 1985), foamed plastic is defined as a plastic in which the apparent density decreases substantially with the presence of numerous cells disposed through its mass. The gas phase in a foamed plastic is generally distributed in cells which are preferably very fine to provide good thermal insulation
Blowing agents produce gas used to generate cells in foamable polymeric materials, for example, to make foamed insulation. Physical blowing agents form cells by a phase change, for example, a liquid may be volatilized or a gas dissolved in a polymer under high pressure. Low boiling (e.g., typically less than 80° C., more typically less than about 50° C.) liquids, particularly chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), have been used throughout the world on a large scale to produce foamed plastics. However, CFCs and HCFCs are linked to the destruction of the earth's protective ozone layer. See Encyclopedia, vol. 2, page 437.
Commercially important liquid blowing agents include aliphatic and cycloaliphatic hydrocarbons and their chloro- and fluoro-derivatives. For example, isomers of pentane, hexane, and heptane are used mainly in the production of very low density polystyrene foam. These liquids tend to be inexpensive and low in toxicity but they are highly flammable. See Encyclopedia, vol. 2, page 434, supra.
Production of cellular plastic products, such as cellular polyurethane elastomers and flexible, semi-rigid or rigid polyurethane foams in the presence of catalysts, blowing agents, processing aids or additives is described in numerous patents and publications in the literature.
A survey of methods of producing cellular polyurethane elastomers, polyurethane foams and polyisocyanurate foams, their mechanical properties and their use can be found, for example, High Polymers, Vol. 14, “Polyurethanes,” Parts I and II by J. H. Saunders and K. C. Frisch (Interscience Publishers, New York 1962 and 1964), Plastics Handbook, Volume VII, “Polyurethanes,” 1st ed. 1966, published by R. Vieweg and A. Hochtlen and 2d ed. 1983, published by G. Oertel (Carl Hanser Verlag, Munich), and “Integral Foams,” published by H. Piechota and H. Rohr (Carl Hanser Verlag, Munich, 1975).
Essentially two types of blowing agents are used to produce cellular polyurethanes: (1) low boiling inert liquids that evaporate under the influence of the exothermic polymerization process, for example, alkanes, such as butane, n-pentane or cyclopentane, halogenated hydrocarbons or halogenated fluorocarbons, such as methylene chloride, dichloromonofluoromethane, and trichlorofluoromethane; and (2) chemical compounds that form gaseous blowing agents by means of a chemical reaction or thermal decomposition, such as isocyanate groups reacted with water to produce carbon dioxide.
The Handbook of Polymeric Foams and Foam Technology, Daniel Klempner and Kurt C. Frisch, ed., (Oxford University Press, 1991), discloses that formation of uniform, fine cellular structure can be obtained by using nucleazites, also referred to as nucleating agents. The Handbook classifies nucleazites into three categories based on their mode of action as follows: (1) gaseous and liquid compounds that produce a supersaturated gas in the foamable composition and which form fine bubbles prior to action by a blowing agent (e.g., carbon dioxide, nitrogen, sodium bicarbonate, citric acid, and sodium citrate), (2) finely dispersed organic, inorganic, or metal powders that form so called “hot spots”, and (3) finely dispersed compounds that provide nucleation centers at which the blowing agent converts to gaseous phase (e.g., talc, silicon dioxide, titanium dioxide, diatomaceous earth, kaolin, etc.).
After the phase out of chlorofluorocarbon (“CFC”) production, many polymeric foams are produced using HCFC-141b (CCl2FCH3) as the blowing agent. With the impending phase out of this blowing agent many producers are looking to use hydrocarbons such as cyclopentane as blowing agents. While foam manufacturers are discovering that they can safely handle the relatively high flammability of these blowing agents, the resultant foams exhibit relatively higher thermal conductivity, a significant drawback to these blowing agents. Foams produced with nonhalogenated blowing agents such as cyclopentane or CO2 (produced in situ via the reaction of water with the isocyanate) typically exhibit thermal conductivities which are 10 to 15 percent higher than those produced with CFCs, HCFCs such as HFC-245fa (CF3CH2CHF2).
Whereas the blowing agent provides the essential volume to form the voids in the foamable resin that become the resultant cells in the finished foam, the nucleating agents provide the initiating sites at which the blowing agent forms the voids. By selection of nucleating agent, one can obtain a foam with fewer relatively larger voids, or a foam with a greater number of relatively smaller voids.
It has been reported that low concentrations of perfluorinated compounds such as C5F12, C6F14, and C5F11NO could be used as a nucleating agent to cause generation of smaller cell sizes in foams. As a result, such foams exhibited lower thermal conductivity. However, due to the long atmospheric lifetimes and high global warming potentials of perfluorinated compounds, their use as nucleating agents is inhibited.
Unsaturated perfluorinated compounds such as HFP dimer [(CF3)2CFCF═CFCF3)] also showed promise as a nucleating agent and offered somewhat better environmental properties as compared to perfluorinated compounds but they have not been commercially adopted for foam blowing applications.
Another alternative is to use partially fluorinated compounds that have been introduced as replacements for CFCs, HCFCs, and PFCs in other applications. Replacement materials such as 3M NOVEC Brand HFE-7100 and HFE-7200 have desirable environmental and toxicological properties but failed to provide acceptable performance as nucleating agents in foams.
The need exists for nucleating agents that exhibit desirable environmental and toxicological properties and function as acceptable nucleating agents and yield higher performing polymeric foams.